Lithium silicate glass ceramics are characterized as a rule by very good mechanical properties, which is why they have been used for some time in the dental field, and there primarily for the preparation of dental crowns and small dental bridges.
U.S. Pat. Nos. 5,507,981 and 5,702,514 describe lithium disilicate glass ceramics which are processed to form dental restorations by pressing in the viscous state. However, the use of a deformable crucible is essential, which makes the processing very expensive.
EP 827 941 and EP 916 625 disclose lithium disilicate glass ceramics which can be given the shape of the desired dental restoration by pressing or machining.
EP 1 505 041 and EP 1 688 398 describe processes for the preparation of dental restorations from lithium disilicate glass ceramics. As an intermediate product a glass ceramic with lithium metasilicate as main crystal phase, which can be very easily machined e.g. by means of CAD/CAM processes, is produced first. This intermediate product is then subjected to a further heat treatment in order to form the desired high-strength lithium disilicate glass ceramic. The heat treatments used during the process are to be chosen such that the formation of undesired crystal phases, such as for example cristobalite, is prevented.
WO 2013/053864 discloses lithium silicate glass ceramics which contain divalent metal oxide and can be processed to form dental restorations by hot pressing as well as by machining.
Glass ceramics are known from WO 2013/164256 which have lithium disilicate as main crystal phase and apatite as a further crystal phase. The glass ceramics are characterized by high chemical stability and can be shaped to form the desired dental restorations by machining or hot pressing.
US 2015/0104655 describes glass ceramics which, depending on the composition and the temperature treatment chosen for the crystallization, can contain lithium disilicate, lithium metasilicate, lithium phosphate, cristobalite, tridymite, quartz or spodumene as crystal phases. The glass ceramics are intended in particular to veneer zirconium oxide ceramics.
However, the machining of the conventional lithium disilicate glass ceramics is possible only with difficulty because of their high strength and it therefore as a rule involves a high wear of the tools used. The likewise possible machining of corresponding lithium metasilicate glass ceramics as precursors is much easier. However, this requires yet a further heat treatment to produce the restoration from high-strength lithium disilicate glass ceramic after the shaping by machining.
There is therefore a need for lithium silicate glass ceramics which are easily machinable and do not require a further heat treatment after this processing in order to give the produced dental restoration the desired mechanical properties. The lithium silicate glass ceramics are to have not only very good mechanical properties, but likewise also very good optical properties, in order that they also fulfil the high aesthetic demands which are made on a restorative dental material.